The present invention relates to a moving low-strength sheet.
A wide variety of sheets are prepared continuously on a commercial scale, often at line speeds of the order of 5,000 feet per minute or ft/min (about 1,500 meters per minute or m/min). When the continuously moving sheet has a very low basis weight or has a loose structure, sheet movement can create a variety of problems associated with the low strength of the sheet.
By way of illustration only, tissue machines support a wet sheet throughout the forming, pressing, and drying processes. Tension is not applied to the sheet until it has been dried. The most common design involves a forming section and a Yankee dryer. The Yankee dryer cylinder is a large-diameter, steam-heated cylinder which serves as the major drying unit. The wet sheet is pressed tightly against the highly polished surface of the cylinder and is transferred to it. The Yankee dryer is enclosed by an air hood and may utilize high-velocity air impingement to increase the drying capacity.
If desired, the sheet may be creped as it leaves the Yankee dryer cylinder to enhance bulk and stretch properties. Creping occurs because of the adherence of the sheet to the dryer cylinder as it comes up against the square edge of a creping doctor.
The creped sheet is very fragile and is subject to a variety of instability and other problems as it travels from the doctor over open spans to and between subsequent rolls and supports, as well as ancillary equipment, before being taken up on a reel drum. The moving sheet imparts momentum to a layer of air adjacent to a sheet surface, giving rise to an entrained boundary layer of air. As the entrained air encounters rolls, supports, and other equipment, turbulent air patterns are established which cause, among other problems, fluttering, which results in tearing of the sheet, and dust generation. These problems, of course, are not limited to creped sheets. They often are associated with any moving low-strength or low basis weight and fragile sheet.